Generally, it is known to produce a transfer sheet comprising a carrier sheet and pigment layer disposed on a surface thereof so that through the application of pressure a portion of the pigment layer is transferred onto a copying sheet.
Basically, two types of transfer sheets have been developed. One type is the so-called two-sheet system which requires two special sheets. The lower surface of one sheet has a layer of a chemical composition disposed thereon and the upper surface of the other sheet has a layer of another chemical composition disposed on it. Pressure on the upper surface of the upper sheet brings the two chemical compositions in pressure contact and results in a visible image on the upper surface of the lower sheet. Neither one of the sheets will produce such a visible image with an untreated sheet. Generally, each sheet for the two-sheet system is treated to have the two chemical compositions on opposite surface portions so that several copies can be produced simultaneously.
The other type of transfer sheet is the so-called single-sheet system and was developed subsequent to the introduction of the two-sheet system. The single-sheet system includes a carrier sheet having a pigment layer disposed on one surface and a transfer layer disposed on the pigment layer. Pressure on the inner surface of the carrier sheet results in a transfer of a portion of the transfer layer to a surface in pressure contact such as a copying sheet and this transfers a portion of the pigment layer to the copying sheet.
These prior art systems have the disadvantage that they can smear because of the pigment layer. Thus, the copying sheet has a smudgy appearance.
Another known single-sheet system includes a carrier sheet, a pigment layer disposed on a surface thereof, and an opaque layer disposed on the pigment layer. Pressure contact on the opaque layer destroys its opacity so that the pigment layer becomes visible.
In order to avoid the disadvantages of the so-called mechanical copying sheets, transfer sheets based on chemical reactions were developed.
Generally, a so-called chemical transfer sheet includes a top sheet having disposed on it lower surface an isolated intermediate dye product dissolved in a solvent and a bottom sheet having disposed on its upper surface a layer of a chemical composition having an acid component. Pressure contact of the dye product and the acid component produces a color change which forms a visible image on the upper surface of the lower sheet. Typically, the intermediate dye product can be crystal violet lactone, malachite green lactone, rhodamine-B lactam, Spyropyrane, and the like. Typically, the acid components can be acid clays such as Attapulgit, kaolin and the like, or phenols having acid terminal groups. The aforementioned patent applications point out the use of a chloride of a metal having an atomic weight between 50 and 66, preferably zinc chloride.
For the chemical compositions, the pressure produced by writing or the like destroys the layer of insulation of the dissolved dye product so that the dye product is transferred to the layer containing the acid component. In particular, a solvent is not needed if the acid component is a metallic chloride.
In a further development of the prior art, the intermediate dye product is micro-encapsulation. Other embodiments include micro-encapsulating the acid component or the solvent.
Transfer sheets using the micro-encapsulated dye product are generally called self-copying sheets and one type is sold under the trade name of Intus-paper. Typically, the underside of the top sheet has the micro-encapsulated dye product and the top surface of the lower sheet has the acid component. Pressure on the upper sheet forces the dye product out of its insulating capsule and into pressure contact with the acid component to produce a color reaction.
Generally, no successful transfer sheet suitable for transferring the visible image onto an untreated surface has been developed.
For a transfer sheet to be useful, it is important that it produce a copy having a sharp contour and definition. Generally, sharp contours have not been possible in the case of chemical transfer sheets, particularly for micro-encapsulated intermediate dye products.